Heat pipe wick fabrication

ABSTRACT

An economical heat pipe wick fabrication technique that yields wicks with fine pores at the liquid/vapor interface and unrestricted fluid flow beneath this interface. The resulting wick may be employed with either high or low thermal conductivity fluids.

United States Patent Arcella et al.

[54] HEAT PIPE WICK FABRICATION [72] Inventors: Frank G. Arcella, BethelPark; Russell E. Brumm, Pittsburgh, both of [73] Assignee: WestinghouseElectric Corporation,

Pittsburgh, Pa.

[22] Filed: March 6, 1970 21 Appl. No.: 17,117

[52] U.S. 29/DIG. 3 [51] Int. Cl. ..B23p 17/00 [58] Field of Search..29/423, 424, DIG. 3

[5 6] References Cited UNITED STATES PATENTS 2,067,746 1/1937 Zabel..29/423 51 Aug. 8, 1972 2,075,637 3/ l 937 Burvenick ..29/423 2,365,67012/ 1944 Wallace ..29/423 2,592,614 4/ l 952 Stoddard ..29/423 2,608,5298/1952 Varian ..29/423 2,694,228 1 [[1954 Mathis ..29/423 2,703,2973/1955 Macbeod ..29/423 2,841,866 7/1958 Schilling ..29/423 PrimaryExaminer-John F. Campbell Assistant Examiner-Donald P. Rooney Attomey-A.T. Stratton and Z. L. Dermer [57] ABSTRACT An economical heat pipe wickfabrication technique that yields wicks with fine pores at theliquid/vapor interface and unrestricted fluid flow beneath thisinterface. The resulting wick may be employed with either high or lowthermal conductivity fluids.

14 Clains, 6 Drawing Figures BACKGROUND OF THE INVENTION AP 2 AP,,.,,,AP

where AP is the pressure differential due to capillary action, AP,,,,,is the pressure drop in the vapor region and AP is the liquid pressuredrop experienced in the wick. Thus, for effective heat pipe operation,the pressure differential due to capillary action, AP must be equal toor exceed the sum of the vapor and liquid pressure drops experienced inthe vapor region and the wick respectively. The greater the difference,the

greater the heat transfer capability of the heat pipe. If

the wick were totally manufactured from fine pore material, the liquidfriction flow factor, AP would be excessively large due to therestrictions to flow.

The prior art has retained the aforementioned critical properties byfabricating channels into the heat pipe walls by a broaching process.The channels, which permit unrestricted fluid flow from the heat pipecondenser to the evaporator section, are covered by a fine mesh screento establish greater capillary wicking forces. Composite wicks have alsobeen fabricated by placing layers of heavy mesh screen to 60 mesh)beneath the liquid/vapor interface layers of fine mesh screen (200 to400 mesh). Another technique comprises the fabrication of open annuluswicks by swaging several turns of screen wound between two copper tubes.The copper tubes are then etched away and the porous rigid wick issinter bonded. Upon insertion into a heat pipe with an open annulusbetween the wick and the heat pipe walls, this wick presents an optimumarrangement for liquid metal charged heat pipes. The first two of theaforementioned techniques have the disadvantages of being bothuneconomical and time consuming and the latter technique is onlysuitable for liquid metal working fluids, since a low thermalconductivity fluid would boil beneath the capillary drawing free wick.Although other wick structures have been fabricated, the three mentionedabove are the ones most frequently employed.

SUMMARY OF THE INVENTION Briefly, this fabrication technique compriseswinding several turns of fine mesh screen onto a cylindrical mandrel.Fine wires are then positioned axially along the mandrel over the finemesh screen. Additional layers of fine mesh screen are wound over theassembly and it is inserted into an outer tube. The final assembly isswaged and the mandrel, wires and outer tube are dissolved away.

This process may also be used to replace the costly broaching processused to fabricate channels in a heat pipe container tube. A similar wickcan be fabricated by utilizing a fine mesh screen/mandrel assembly whichis covered with axial wires and inserted into an outer tube and swaged.Dissolving away the mandrel and wires leaves a tight homogeneous,channeled wick.

Thus, the present fabrication technique provides fine pore sizes at thewick liquid/vapor interface and channels immediately below thisinterface for unrestricted fluid flow (low friction flow). The resultingprocess is economical and produces a wick that can be employed witheither high or low thermal conductivity fluids.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of anexemplary embodiment of this invention, reference may be had to theaccompanying drawings, in which:

FIG. 1 is an isometric view of a heat pipe, having a portion thereof cutaway for clarity, and including an enlarged longitudinal sectional viewof a portion of a heat pipe wick constructed in accordance with theteachings of this invention;

FIG. 2 is an isometric view of a heat pipe wick assembly broken away inlayers, and is illustrative of an interim stage of the wick assemblyduring fabrication of the heat pipe wick of FIG. 1;

FIG. 3 is a cross-sectional view of the heat pipe wick assembly of FIG.2 and is taken along the lines III-III thereof;

FIG. 4 is a cross-sectional view of the heat pipe wick of FIG. 1;

FIG. 5 is an isometric view of a heat pipe wick assembly broken away inlayers and is illustrative of an interim stage of the wick assemblyduring fabrication of another embodiment of a heat pipe wick constructedin accordance with the teachings of this invention; and

FIG. 6 is a cross-sectional view of another embodiment of a heat pipewick constructed in accordance with the teachings of this invention andis illustrative of the final wick derived from the interim assemblyillustrated in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the heat pipeillustrated in FIG. I, it will be appreciated that a heat pipe 10constructed in accordance with the principles of this invention includesan evacuated chamber 12 whose inside walls are lined with a capillarystructure, or wick 30, that is saturated with a volatile fluid. Theoperation of a heat pipe combines two familiar principles of physics;vapor heat transfer and capillary action. Vapor heat transfer serves totransport the heat energy from the evaporator section 14 at one end ofthe pipe to the condenser section 16 at the other end. Capillary actionreturns the condensed working fluid back to the evaporator section 14 tocomplete the cycle.

The working fluid absorbs heat at the evaporator section 14 and changesfrom its liquid state to a gaseous state. The amount of heat necessaryto cause this change of state is the latent heat of vaporization. As theworking fluid vaporizes, the pressure at the evaporator section 14increases. The vapor pressure sets up a pressure differential betweenthe ends of the heat pipe, and this pressure differential causes thevapor, and thus the heat energy, to move towards the condenser section16. When the vapor arrives at the condenser section 16, it

is subjected to a temperature slightly lower than that of the evaporatorsection 14 and condenses, thereby releasing the thermal energy stored inits heat of vaporization at the condenser section 16 of the heat pipe.As the vapor condenses the pressure at the condenser section 16decreases so that the necessary pressure differential for continuedvapor heat flow is maintained.

Movement of the fluid from the condenser section 16 back to theevaporator section 14 is accomplished by capillary action within thewick 30 which connects the condenser 16 to the evaporator 14.

As is known, the driving force that causes the liquid to move throughthe capillary is the surface tension of the liquid. When a fluid isplaced in a compatible vessel, that is a vessel composed of a materialthat the fluid wets well, there is an attractive force between the fluidand the walls of the vessel. This force combines with the surfacetension in such a way as to move the liquid towards the unfilled portionof the vessel. If the vessel is a capillary of small diameter, such asthe wick 30, this force, called capillary attraction, can be largecompared with the mass of fluid in the capillary. The resulting forceswill thus cause the liquid to pump itself through the wick indefinitelyin the absence of other forces. It is the use of vapor pressure andcapillary action that enables the heat pipe to operate as aself-contained heat pump.

For liquid metal working fluids the wick 30 must retain several criticalproperties. For example the wick liquid/vapor interface 32 must possessextremely small pore sizes for optimum capillary drawing forces. Thiscan be seen from the following equation;

AP Z AP AP where AP, is the pressure differential due to capillaryaction, AP is the pressure drop in the vapor region and AP, is theliquid pressure drop experienced in the wick. Thus, for effective heatpipe operation, the pressure differential due to capillary action, APmust equal or exceed the sum of the vapor, AP,,,,,,, and liquid, APpressure drops experienced in the vapor region and wick respectively. Ifthe wick were totally manufactured from fine pore material, the liquidfriction flow factor, A t" would be excessively large due to therestrictions to flow. A more detailed explanation of heat pipe operationis presented in the May, 1968 issue of Scientific American, in anarticle entitled 'Ihe Heat Pipe, by Y. Eastman.

This invention provides an economical heat pipe wick fabricationtechnique that retains the aforementioned critical properties byproviding fine pore sizes at the liquid/vapor interface 32 and channels34 immediately below this interface for unrestricted fluid flow (lowfriction flow).

Referring now to the heat pipe wick assembly illustrated in FIGS. 2 and3, it will be appreciated that a heat pipe wick assembly 35 constructedin accordance with the principles of this invention includes severalturns of fine mesh screen 36, from approximately 200 to 500 mesh (orhigher mesh), desirably constructed from a material that is relativelyresistant to being etched away in the chosen etchant specified below,such as 304 stainless steel or pressed and sintered felt metal, whichare wound onto a central mandrel 38,

desirably constructed from a material that can be etched away in thechosen etchant. The mandrel 38 may be designed so as to assume anydesired shape depending upon the desired shape of the wick beingfabricated, but it is to be understood that the shape of the mandrel isnot to be limited to the hollow circular cylindrical configurationillustrated by reference character 38. Fine wires 40, desirablyconstructed from a material that will dissolve in the chosen etchant,are laid axially along the mandrel 38 over the fine mesh screen 36. Thesize of the wires will depend upon the working fluid being employed inthe heat pipe. For example, for a working fluid such as sodium thediameter of the wires may vary approximately from 15 to 25 mills.Additional layers of fine mesh screen 42, approximately from 200 to 400mesh, desirably formed from the same material as the mesh screen 36, arewound over the assembly 35 and inserted into an outer tube 44, which isconstructed from a material having the same characteristics as that ofthe central mandrel 38, and which closely receives the assembly 35 inthe center thereof. The assembly is then shaped by swaging or by anyother process that uniformly constricts the interface between the twomesh layers 36 and 42 so that they closely conform to the axially laidwire surface at the wire/mesh interface and form a continuous meshcross-section at the mesh/mesh interface. Such a process may also beaccomplished by expanding the central mandrel 38 against the fixed outertube 44 by internally pressurizing and/or heating the central mandrel38. The mandrel 38, wires 40 and outer tube 44 are then dissolved awayin a suitable etching solution such as nitric acid where the componentsto be etched away are constructed from copper, or sodium hydroxide maybe used where the components to be etched away are constructed fromaluminum. It is to be understood that any other etchant may be used thatwill suitably react with the components to be dissolved away withoutdissolving the mesh screen.

Referring now to FIG. 4, it will be observed that a free standing wick46 with channels 48 just beneath the evaporator surface 50 results fromthis process. It can also be seen that this process can easily replacethe costly breaching process previously used to fabricate channels in aheat pipe container tube. This may be accomplished by the assemblyillustrated in FIG. 5, which is similar to the assembly illustrated inFIG. 2. This assembly includes several turns of fine mesh screen 52,formed from a material having the same characteristics as the meshscreen 36, which are wound around a central mandrel 54, having the samecharacteristics as the mandrel 38. Fine wires 56, which are formed froma material having the same characteristics as that of the wires 40, arelaid axially along the mandrel 54 over the fine mesh screen 52. Theassembly is then inserted into an outer tube 58, constructed from amaterial that is relatively resistant to the chosen etchant mentionedbelow. The entire assembly is then swaged, or by any other processuniformly constricted, so that the mesh screen 52 conforms to the shapeof the axially laid wires 56 and outer tube 58 at the wire/mesh andtube/mesh interface respectively. The mandrel 54, and wires 56 are thendissolved away in a suitable etching solution such as nitric acid wherethe components to be etched away are constructed from copper, or sodiumhydroxide where the components to be etched away are constructed fromaluminum. It is to be understood that any other etchant may be used thatwill suitably react with the components to be dissolved away withoutdissolving the mesh screen 52 and outer tube 58. Referring now to FIG. 6it will be observed that the resulting structure is a tight homogeneous,channeled wick similar to the wick fabricated by the aforementionedbroaching process.

We claim:

1. A process for fabricating a heat pipe wick having channels justbeneath the fine pore surfaces, which comprises winding several turns offine mesh screen over a mandrel, placing a plurality of fine wiresaxially along said mandrel over said fine mesh screen and inserting theresulting assembly into an outer tube which closely receives saidassembly in the center thereof, constricting the final assembly so thatthe mesh screen conforms to the shape of the axially laid wires andouter tube at the wire/mesh and tube/mesh interface respectively andthen dissolving away said mandrel and said wires.

2. The process for fabricating a heat pipe wick of claim 1 wherein saidmandrel and said wires are formed from copper and said fine meshmaterial and said outer tube are chosen to be insoluble in nitric acid.

3. The process for fabricating a heat pipe wick of claim 2 wherein saiddissolving step comprises etching away said copper wires and said coppermandrel in nitric acid.

4. The process for fabricating a heat pipe wick of claim 1 wherein saidmandrel and said wires are formed from aluminum and said fine meshmaterial and said outer tube are chosen to be insoluble in sodiumhydroxide.

5. The process for fabricating a heat pipe wick of claim 4 wherein saiddissolving step comprises etching away said aluminum wires and saidaluminum mandrel in sodium hydroxide.

6. The process for fabricating a heat pipe wick of claim 1 wherein saidfine mesh screen is constructed from a material selected from the groupconsisting of 304 stainless steel and pressed and sintered felt metal.

7. The process for fabricating a heat pipe wick of claim 1 wherein saidconstricting step comprises swaging said final assembly.

8. The process for fabricating a heat pipe wick of claim 1 wherein saidconstricting step comprises expanding said mandrel against said rigidouter tube.

9. The process for fabricating a heat pipe wick of claim 1 includingwinding several turns of fine mesh screen over said axially laid wires,constricting said final amembly so that the mesh screen closely conformsto the axially laid wire surface at the wire/mesh interface and forms acontinuous mesh cross-section at the mesh/mesh interface and dissolvingaway said outer tube concurrently with said wires and said mandrel.

10. The process for fabricating a heat pipe wick of claim 9 wherein saidmandrel, said wires and said outer tube are formed from copper and saidfine mesh material is chosen to be insoluble in nitric acid.

11. The process for fabricating a heat pipe wick of claim 10 whereinsaid dissolving step comprises etching copper outer tube in nitric acid.

12. The process for fabricating a heat pipe wick of claim 9 where saidmandrel, said wires and said outer tube are formed from aluminum andsaid fine mesh material is chosen to be insoluble in sodium hydroxide.

13. The process for fabricating a heat pipe wick of claim 12 whereinsaid dissolving step comprises etching away said aluminum wires, saidaluminum mandrel and said aluminum outer tube in sodium hydroxide.

14. The process for fabricating a heat pipe wick of claim 9 wherein saidfine mesh screen is constructed from a material selected from the groupconsisting of 304 stainless steel and pressed and sintered felt metal.

l t t

1. A process for fabricating a heat pipe wick having channels justbeneath the fine pore surfaces, which comprises winding several turns offine mesh screen over a mandrel, placing a plurality of fine wiresaxially along said mandrel over said fine mesh screen and inserting theresulting assembly into an outer tube which closely receives saidassembly in the center thereof, constricting the final assembly so thatthe mesh screen conforms to the shape of the axially laid wires andouter tube at the wire/mesh and tube/mesh interface respectively andthen dissolving away said mandrel and said wires.
 2. The process forfabricating a heat pipe wick of claim 1 wherein said mandrel and saidwires are formed from copper and said fine mesh material and said outertube are chosen to be insoluble in nitric acid.
 3. The process forfabricating a heat pipe wick of claim 2 wherein said dissolving stepcomprises etching away said copper wires and said copper mandrel innitric acid.
 4. The process for fabricating a heat pipe wick of claim 1wherein said mandrel and said wires are formed from aluminum and saidfine mesh material and said outer tube are chosen to be insoluble insodium hydroxide.
 5. The process for fabricating a heat pipe wick ofclaim 4 wherein said dissolving step comPrises etching away saidaluminum wires and said aluminum mandrel in sodium hydroxide.
 6. Theprocess for fabricating a heat pipe wick of claim 1 wherein said finemesh screen is constructed from a material selected from the groupconsisting of 304 stainless steel and pressed and sintered felt metal.7. The process for fabricating a heat pipe wick of claim 1 wherein saidconstricting step comprises swaging said final assembly.
 8. The processfor fabricating a heat pipe wick of claim 1 wherein said constrictingstep comprises expanding said mandrel against said rigid outer tube. 9.The process for fabricating a heat pipe wick of claim 1 includingwinding several turns of fine mesh screen over said axially laid wires,constricting said final assembly so that the mesh screen closelyconforms to the axially laid wire surface at the wire/mesh interface andforms a continuous mesh cross-section at the mesh/mesh interface anddissolving away said outer tube concurrently with said wires and saidmandrel.
 10. The process for fabricating a heat pipe wick of claim 9wherein said mandrel, said wires and said outer tube are formed fromcopper and said fine mesh material is chosen to be insoluble in nitricacid.
 11. The process for fabricating a heat pipe wick of claim 10wherein said dissolving step comprises etching away said copper wires,said copper mandrel and said copper outer tube in nitric acid.
 12. Theprocess for fabricating a heat pipe wick of claim 9 where said mandrel,said wires and said outer tube are formed from aluminum and said finemesh material is chosen to be insoluble in sodium hydroxide.
 13. Theprocess for fabricating a heat pipe wick of claim 12 wherein saiddissolving step comprises etching away said aluminum wires, saidaluminum mandrel and said aluminum outer tube in sodium hydroxide. 14.The process for fabricating a heat pipe wick of claim 9 wherein saidfine mesh screen is constructed from a material selected from the groupconsisting of 304 stainless steel and pressed and sintered felt metal.